The effect of two different monovalent alginate counterions has been studied in Ca 2 + -mediated sol/gel transitions and in Ca 2 + -gels at apparent equilibrium for alginates with different chemical composition and sequence. K-alginates were generally found to undergo faster sol/gel transitions compared to the Na-alginate. At apparent equilibrium and at low Ca 2 + concentration, Na-alginate gels showed reduced elastic moduli compared to the K-alginates. The relative differences both in sol/gel transition kinetics and modulus at apparent equilibrium increased with increased content of guluronic acid residues in the alginate sample. Increasing the amount of internally released Ca 2 + from 15 to 30 mM resulted in equal elastic properties at apparent equilibrium, suggesting that dissimilarities in ion exchange reactions between Ca 2 + /Na + and Ca 2 + /K + is the main driving force. The specific effect of one of the counterion could be reversed by adding increasing amounts of the other. Selectivity coefficient determinations for alginates revealed a slight enhanced binding of Na + compared to K + , most probably due to the guluronate residues.